This application is a competitive continuation of R01 AI49730-03. There are a number of advantages to the use of peptide and small well-defined oligosaccharide (OS) antigens for subunit vaccine design. For example, chemical purity and safety, ease of production, cost, stability and mutability. However, peptidic and OS antigens require conjugation to T cell carrier proteins for efficient delivery to the immune system. The hepatitis core platform is a particulate carrier capable of eliciting high titer antibody responses to weak "hapten-like" peptidic and OS antigens incorporated onto core particles. During the course of this grant we have used the hepatitis B core (HBcAg) as a carrier platform for P. falciparum malaria circumsporozoite (CS) neutralizing repeat sequences (i.e., NANPn). The hybrid HBcAg-CS repeat particles are highly immunogenic in mice and are currently in phase 1/11clinical trials. Although the HBcAg is highly immunogenic, we have identified a number of theoretical and practical limitations to the use of the HBcAg as a vaccine platform. To address these limitations we are developing the core protein from the woodchuck hepadna virus (WHcAg) as a particulate carrier platform for several reasons: (1) The WHcAg is equally or more immunogenic than the HBcAg at the B cell and T cell levels; (2) the use of the WHcAg does not compromise the anti-HBc diagnostic assay because WHcAg and HBcAg are not crossreactive at the antibody level; (3) no pre-existing antibody to the WHcAg is present unlike for the HBcAg; (4) immune tolerance to HBcAg in chronic HBV carriers (300-400 million worldwide) can be circumvented by the use of the WHcAg platform because WHcAg and HBcAg are only partially crossreactive at the CD4+ T cell level; and (5) we have developed a WHcAg-based combinatorial technology that is more versatile than the existing HBcAg technology in terms of accommodating the insertion of a greater variety of foreign epitopes. For example, we have modified the WHcAg gene to create libraries of 17 insertion sites and 22 C-terminal modifications. Combining these libraries has allowed us to successfully insert 22 of 24 attempted foreign epitopes into the WHcAg platform. We propose to continue to expand the WHcAg combinatorial technology and perform biochemical/structural analysis of hybrid-WhcAg particles in an attempt to correlate structure and function (Aim 1); characterize the immune response to hybrid-WHcAg particles in vitro and in vivo in small animal models including challenge experiments to determine the protective efficacy of hybrid-WHcAg vaccine candidates if warranted (Aim 2); and extend the WHcAg platform to accommodate non-linear larger protein and carbohydrate antigens (Aim 3). A number of unique immunologic characteristics of the WHcAg and of the hybrid-WHcAg particles produced to date suggest that this particulate platform will be useful as a means of delivering a variety of medically relevant antigens to the immune system.